This invention generally relates to a process and apparatus for the hydrogenation of polymers. More particularly, the present invention relates to a process and apparatus for the hydrogenation of polymers under supercritical conditions.
Processes for the hydrogenation of polymers in solution are typically characterized by very long reaction cycle times, on the order of several hours. This low productivity results in economic in efficiency. Hydrogenation of polymers has traditionally been conducted as a batch process, with hydrogen gas being pressurized into solution to reach the polymer. A catalyst is also typically employed. Although the catalyzed reaction between hydrogen and the double bonds of a polymer is quite rapid, the apparent rate of the reaction is relatively slow due to two factors. First, the rate of mass transfer of hydrogen gas across the gas/liquid interface is slow—even at high pressures, with vigorous agitation. Second, hydrogen solubility is generally poor, in the solvent systems typically employed such that, even with exceptional interphase transfer rates, the concentration of hydrogen available (dissolved) in the liquid phase is always quite low. While it might be proposed to compensate for the low solubility of hydrogen by operating at higher hydrogen gas pressures, the resulting increase in the rate of reaction would be modest. Additionally, the capital cost and safety concerns associated with the large batch reactors required for such conditions would be undesirable.
To overcome problems encountered in the prior art, the present invention investigates hydrogenating polymers under supercritical conditions. With reference to FIG. 1, it can be see that, for a pure component that is subject to increases in temperatures and pressures, there exists a “critical point” above which the pure component enters the supercritical phase and exhibits supercritical phase behavior. The critical point represents the intersection of the critical pressure and critical temperature lines in the phase diagram, and, thus, a pure component or multi-component mixture is in the supercritical phase when “above the critical point.” The “critical temperature” is the temperature above which no amount of pressure increase will cause the component to liquefy from the supercritical phase. Likewise, the “critical pressure” is the pressure at which no amount of temperature increase will cause the component to evaporate from the supercritical phase to the gas or other phase.
It is well established that mixtures of gases and liquids that are heated and pressurized above the critical point for the specific mixture become a single phase, with essentially complete miscibility. Additionally, under supercritical conditions the viscosity and density of the mixture is typically lower than the previous phase, and the molecular diffusivity is greatly increased, as is the thermal conductivity of the mixture. Thus, the present invention investigates the hydrogenation of polymers under supercritical conditions, because supercritical mixtures do not appear to suffer from the problems associated with the hydrogenation of polymers in two-phase (gas/liquid), heterogeneous systems. Particularly, under supercritical conditions, there is no need to pressurize hydrogen gas into a solvent system, because each component is in a single supercritical phase, and hydrogen could be more efficiently employed. Mass transfer problems, as mentioned above, would likewise be eliminated because interphase boundaries cease to exist at supercritical conditions.
Thus, it is believed that, under supercritical conditions, the reactions involved in the hydrogenation of polymers would be limited only by the rapid kinetics of the reaction between hydrogen and the double bonds of the polymer, in light of the characteristics of mixtures in the supercritical phase. However, supercritical operation is normally associated with extreme levels of pressure and temperature, and these operating conditions usually mandate high capital investments and operating costs. Safety is also an important issue, especially when materials such as hydrogen and typical polymer solvents are involved.
The present invention investigates a very practical way to apply the strong merits offered by supercritical processing to the hydrogenation of polymers, while minimizing capital investment, operating costs, and safety concerns, relative to the subcritical technology previously considered.